I've been listening to a pair of Voigt Pipes w/ Lowther PM6A, and I've decided I want to try adding a horn superwteeter. The Lowther's are 15 ohms, and I use the 16 ohm taps on my SET amps.

Anway, I was wondering if someone could help me out with recommendations (in any price range), or at the very least teach me what impedance + efficiency I should be looking for in the tweeter... and perhaps how to determine which cap values to experiment with?

1st of all I have been very happy with the Fostex T90a and T900A. The T900A is a little bit nicer, but much more expensive. Either will work well.

In theory (that is to say, I have not tried it yet) a .56 mh coil in series with the PM6A combined with a zobel using a 1 uf cap and a 22.5 ohm resistor will cross you over around 11 kHz.

Use a 2 uf cap with a .2mh coil for the tweeter crossover (12 db, with the inductor in parrellel with the driver, and the cap between the amp and inductor). Then use an L pad to adjust the volume. Personally I would not worry about the fact this is 8 ohm driver as compared to the 15 ohm Lowther, but if you want to worry feel free to go to step 2.

If you enjoy the challenge you can convert these values to a 16 ohm crossover and then add an 8 ohm resistor in series with the driver, between the driver and the inductor. Then you have to develop a proper pad network to bring the efficiency down from 106 db of the tweeter to the 97 db of the Lowther. Keep in mind that the Lowther is 97 db @ 3.87 volt (as it is 15 ohm) whereas the Fostex is 106 db @ 2.83 volts. Math is not hard, and you should enjoy it.

All that being said, these mesh very well with Lowthers. Trying ribbons in the past found them lacking in dynamics, with a kind of boring sound as a result.